1. Field of the Invention
The present invention relates generally to the control of heating, ventilating and air conditioning (HVAC) systems, and more particularly to a method and system for providing adaptable control of temperature and humidity for minimizing mold and mildew while reducing energy consumption.
2. Background Art
U.S. Pat. No. 5,170,935 issued to Federspiel et al. on Dec. 15, 1992 discloses an adaptable control of HVAC systems which regulates environmental conditions within an enclosed area. The apparatus and method described measures selected environmental variables in the enclosed area, calculates a value of a comfort index which is a function of the values of the selected environmental variables and a plurality of parameters that predicts a thermal sensation rating of an occupant. The system receives a sensation rating from the occupant and compares it to the predicted thermal sensation rating to determine a difference. A parameter estimation process estimates the value of at least one parameter and changes the value to reduce the difference between the sensation ratings if necessary or desired. The process is repeated until the sensation difference is substantially eliminated. Federspiel '935 recognizes the need for thermal comfort and point out that thermal comfort is primarily dependent upon whole body thermal sensation which is a function of six variables including air temperature, humidity, air velocity, clothing insulation, bodily heat production rate, and mean radiant temperature. Federspiel '935 teaches a direct contact by a human occupant to determine the occupants perceived comfort level.
U.S. Pat. No. 4,889,280 issued to Grald et al. on Dec. 26, 1989 discloses a temperature and humidity auctioning control adapted to be connected to a thermostat control which includes a temperature sensor that provides a sensed temperature signal. The auctioning control for humidity and temperature is completed without a separate humidity controller and provides humidity control information to the thermostat. A space temperature setpoint is lowered by a precise amount needed to achieve proper humidity control.
Various methods and devices have been used to control a space environment by focusing on control of one or a combination of a cooling zone, a dehumidifying zone and a fan or air flow zone. By way of example, U.S. Pat. No. 4,271,898 issued to Freeman on Jun. 9, 1981 discloses an economizer comfort index control which includes a control relay activated when the thermostat selector switch is in the cool position and the fan selector switch in on to cause the blower motor to run at a high speed while the compressor is running and at a low speed while the compressor is not running. A relative humidity controller makes the HVAC system responsive to relative humidity as well as temperature for maintaining an acceptable nighttime comfort index while reducing energy usage by the HVAC system. In other words, humidity control is essentially accomplished by increasing speed control of the fan rather than lowering the temperature of the space with the thought of conserving energy.
Occupancy-sensing setback controllers have been used in hotel rooms and other applications since the 1970's. The extent of the setback is limited in coastal and sub-tropic climates due to the potential for mold and mildew damage caused by high relative humidity. There has been developed and is now in production a microcomputer-based, occupancy-sensing setback controller which senses relative humidity in addition to temperature. Because of the power of the onboard microcomputer, a psychrometric algorithm using the thermodynamic states of temperature and relative humidity has been developed. This algorithm can maximize the extent to which the HVAC can remove moisture from the room when it is damp, thereby reducing furniture, fixtures and equipment damage, yet when the room is dry, an energy-saving setback temperature cycle can be utilized.
The typical occupancy-sensing setback controller operates with a very simple control algorithm based on temperature only. When the room is unoccupied (door closed, no motion detected), the HVAC's conventional thermostat is disabled until the room temperature reaches either the summer or winter setback temperature selected at the setback controller. When this setback temperature is reached, the HVAC is enabled until the room temperature decreases (or increases) approximately 2.degree. F. Thus, if the room heat load is such that the room heat load is such that the room never reaches the setback temperature, then the HVAC remains disabled until the room is reoccupied (door closed, motion detected). This period will often occur when room conditions are very favorable to the growth of mold and mildew.
More advanced occupancy-based setback controllers are designed with an on-board microcomputer which greatly increases the flexibility that can be designed into the control algorithm. Some of the features include the ability to differentiate between interior and exterior door control responses, provide room refresh cycling to avoid stagnant air build-up during the room occupant's absence and the ability to bypass the controller in a non-regressive fashion. An automatic unsold mode which allows increasing the summer setback temperature to 85.degree. F. (and a winter setback temperature of 55.degree. F.) is also available. When servicing the unsold room, the maid blocks the door open. This allows the HVAC to operate during servicing but retains the unsold setback temperature when the maid leaves the room and closes the door. There is also an out-of-service mode available which has temperature setbacks of 96.degree. F. summer (40.degree. F. winter). This mode is for rooms that are not used due to maintenance or low seasonal occupancy. Although the auto unsold and the out-of-service modes save additional energy, they also will encounter extended periods when the room conditions will be very favorable to the growth of mold and mildew. Thus, an algorithm capable of avoiding the environmental regions favorable to the growth of mold and mildew is needed for many property locations.
The staff at the University of Florida's Institute of Food and Agricultural Sciences has broadly defined the "mold and mildew zone" as the psychrometric region above 72.degree. F. and above 60% relative humidity. Efforts to avoid the mold and mildew zone by lowering the temperature below 72.degree. F. can also be unsatisfactory if the outside dew point temperature is greater than the room temperature. If, when the door is opened, the in-rushing outside air is at a high dew point temperature, condensation will occur on the room furnishings, walls and windows. This moisture will become imbedded and take a long time to be removed when the room is warmed, thus becoming another incubation site for mold and mildew. In addition, the condensation will cause corrosion on brass and other metallic surfaces, particularly in coastal regions. There is therefore a need for an effective control algorithm that uses the HVAC to escape the mold and mildew comfort zone without reducing room temperature to the point where condensation occurs. Using an occupancy-sensing setback controller to reduce electrical costs when the room is unoccupied and/or unsold can subject the room to nearly ideal conditions for mold and mildew growth, unless the control algorithm is capable of accounting for room relative humidity as well as temperature.